Display apparatus

ABSTRACT

A display device having a scanning line, a data line, a power supply line, and a pixel. The pixel having a first transistor supplied with a selecting pulse of a scanning signal, a holding capacitor having a first electrode and a second electrode that holds an image signal from the data line and the first thin film transistor. The pixel also having a second transistor controlled by the image signal, a gate of the second transistor being electrically connected to the second electrode, and a luminescent element provided between a pixel electrode and an opposite electrode opposed to the pixel electrode driven by current that flows between the pixel electrode and the opposite electrode. A potential of the gate electrode of the second transistor being able to be shifted by supplying to the first electrode of the holding capacitor with a predetermined signal after the selecting pulse becomes non-selective.

This is a Continuation of application Ser. No. 12/155,813, filed Jun.10, 2008, which in turn is a Continuation of application Ser. No.11/505,459, filed Aug. 17, 2006, which in turn is a Continuation ofapplication Ser. No. 10/267,834, filed Oct. 10, 2002, now U.S. Pat. No.7,221,339, which in turn is a Division of application Ser. No.09/171,224, filed Oct. 16, 1998, now U.S. Pat. No. 6,522,315, which inturn is a U.S. National Phase of PCT/JP98/00656, filed Feb. 17, 1998.The disclosures of the prior applications are hereby incorporated byreference herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an active matrix type display apparatusincluding luminescent elements such as EL (Electro-luminescence)elements or LED (Light Emitting Diode) elements which emit light bydriving current flowing in thin films of organic semiconductors or thelike, and also including thin film transistors (hereinafter TFT's) tocontrol the emitting operation of these luminescent elements. Moreparticularly, the present invention relates to a technique of drivingeach element formed in this type of display apparatus.

2. Description of the Related Art

Active matrix type display apparatuses incorporating luminescentelements of a current controlling type, such as EL elements and LEDelements, have been proposed. Since any luminescent element employed inthis type of display apparatus emits by itself, there are advantages inusing no back-light and in having a minimal dependence on the viewingangle and the like.

FIG. 31 is a block diagram illustrating an active matrix type displayapparatus incorporating organic thin film EL-elements of an electriccharge filling type, as an example of these types of displayapparatuses. In the display apparatus 1A shown in this figure, aplurality of scanning lines “gate”, a plurality of data lines “sig”extending in a direction that intersects the direction in which thescanning line “gate” extend, a plurality of common power supply lines“com” extending parallel to the data lines “sig”, and a plurality ofpixels 7 located at the intersections of the data lines “sig” and thescanning lines “gate” which are formed on a transparent substrate.

Each pixel 7 comprises a first TFT 20 in which a scanning signal issupplied to the gate electrode (a first gate electrode) through thescanning gate, a holding capacitor “cap” which holds an image signalsupplied from the data line “sig” via the first TFT 20, a second TFT 30in which the image signal held by the holding capacitor “cap” issupplied to the gate electrode (a second gate electrode), and anluminescent element 40 (indicated as a resistor) into which the drivingcurrent flows from the common power supply line “com” when the element40 is electrically connected to the common power supply line “com”through the second TFT 30.

In the above display apparatus 1A, both the first TFT 20 and the secondTFT 30 are conventionally formed, as with an N channel type TFT or a Pchannel type TFT, as shown in an equivalent circuit diagram of FIG. 32,from the viewpoint of simplifying the production process, for example,in the case of an N channel type. Taking the N channel type as anexample, as shown in FIGS. 33 (A) and (B), when the high potential imagesignal “data” is written into the holding capacitor “cap” from the dataline “sig”, while the scanning signal “Sgate” supplied through thescanning line “gate” has become higher in potential to turn the firstTFT 20 “on”, the second TFT 30 is held in the “on” state. Consequently,in the luminescent element 40, the driving current keeps flowing from apixel electrode 41 to an opposite electrode “op” in the directionindicated by the arrow “E” and consequently, the luminescent element 40keeps emitting (the “on” state). On the other hand, when the imagesignal “data”, which is lower than the intermediate between thepotential of the common power supply line “com” and the potential of theopposite electrode “op”, is written into the holding capacitor “cap”from the data line “sig”, while the scanning signal “Sgate” suppliedthrough the scanning line “gate” has become higher in its potential toturn the first TFT 20 “on”, the second TFT 30 is turned “off” andconsequently, the luminescent element 40 is turned “off” (the “off”state).

In the above display apparatus 1A, a semiconductor thin film, aninsulating thin film, an electrode, etc., which constitute each element,are formed by thin films deposited on the substrate. Considering theheat resistance of the substrate, a low-temperature process is oftenused to form the thin films. Therefore the quality of the thin film ispoor, as is evidenced by the frequent defects caused by aphysical-property difference between a thin film and a bulk, whichresult in problems such as an electrical breakdown, and whereintime-degradation is apt to arise in the TFT and similar devices.

In the case of a liquid crystal display apparatus incorporating liquidcrystals as light modulation elements, although it also uses the thinfilms, time-degradation can be suppressed not only in the liquid crystalbut also in the TFT, because the light modulation element is driven byAC power. On the other hand, in the display apparatus 1A incorporatingluminescent elements of the current controlling type, time-degradationis more often encountered in the TFT than in the liquid crystal displayapparatus insofar as the apparatus is essentially driven by D.C. power.Although improvements have been made in the structure of the TFT and theprocess techniques in the display apparatus 1A, incorporatingluminescent elements of the current controlling type, they do not yetseem to be improved enough.

In the case of incorporating the liquid crystals as the light modulationelements, the power consumption is small because the light modulationelement is controlled by the voltage which causes the current flow ineach element to be only momentary. On the other hand, in the displayapparatus 1A incorporating luminescent elements of the currentcontrolling type, a constant driving current is required to keep theluminescent element “on”, and this results in high power consumption andthe risk of the frequent occurrence of electrical breakdown andtime-degradation.

Further, in the liquid crystal display apparatus, the liquid crystal canbe AC-driven by one TFT per one pixel. On the other hand, in the displayapparatus 1A incorporating luminescent elements of the currentcontrolling type, the luminescent element 40 is DC-driven by two TFTs20, 30 per one pixel. This raises the driving voltage, and exacerbatesthe aforementioned problems, such as electrical breakdown andtime-degradation. For example, as shown in FIG. 33 (A), the gate voltage“Vgsw” of the first TFT, when selecting a pixel, corresponds to thepotential difference between the potential equals to the higherpotential of the scanning signal “Sgate” and the potential of thepotential-holding electrode “st” (the potential of the holding capacitor“cap” or the potential of the gate electrode of the second TFT 30).Therefore when the potential of the potential-holding electrode “st”and, hence, the gate voltage “Vgcur” of the second TFT 30 are raised tomake the luminescent element 40 emit in a high luminance, the gatevoltage “Vgsw” of the first TFT 20 is lowered correspondingly.Therefore, the greater amplitude of the scanning signal “Sgate” has tobe employed, requiring the higher driving voltage in the displayapparatus 1A. Besides, in the aforementioned display apparatus 1A, sincewhen the luminescent element 40 is “off”, the potential of the imagesignal “data” is made lower than the intermediate potential between thepotential of the common power supply line “com” and the potential of theopposite electrode “op” in order to turn the second TFT 30 “off”, thereis another problem of increased amplitude of the image signal “data”.Accordingly, in this display apparatus 1A, special consideration for thepower consumption and the withstanding of voltage of the TFT, etc. isneeded compared to the liquid crystal display apparatus. However, theconventional display apparatus 1A has not been provided with thesufficient consideration of these factors.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a displayapparatus, which improves display image quality as well as suppressespower consumption, electric breakdown and deterioration with time byreducing the driving voltage, relying upon a driving method which takesinto account the conduction types of TFTs used for controlling emissionoperations of the current-driven light-luminescent elements so as toreduce the driving voltage, which improves both the display imagequality and characteristics such as power consumption, breakdown andtime-degradation.

To accomplish the aforementioned object, the present invention inproposes a display apparatus comprising, arranged on a substrate, aplurality of scanning lines, a plurality of data lines intersecting thescanning lines, a plurality of common power supply lines, a plurality ofpixels formed by the scanning lines and the data lines in a matrix form,each of said pixels comprising a first TFT having a first gate electrodewhich is supplied with a scanning signal through the scanning line, aholding capacitor which holds an image signal supplied by the data linethough the first TFT, a second TFT having a second gate electrode whichis supplied with the image signal held by the holding capacitor, anemitting thin film, which emits light due to the driving current whichflows between the pixel electrode and an opposite electrode, which isopposed to the pixel electrode with the emitting thin film providedtherebetween, when the pixel electrode is electrically connected to thecommon power supply line through the second TFT, wherein the potentialof the common power supply line is set at a lower level than that of theopposite electrode when the second TFT is of an N channel type.

In the display apparatus according to the present invention, since thegate voltage of the second TFT at “on” state corresponds to thedifference between the potential of gate electrode (the potential of theimage signal) and one of the potential of the common power supply lineand the pixel electrode, the gate voltage of the second TFT is arrangedso as to correspond to the potential difference between the common powersupply line and the potential-holding electrode by optimizing relativepotential values between the common power supply line and the oppositeelectrode of the luminescent element according to the conduction type ofthe second TFT. For example, when the second TFT is of an N channeltype, the potential of the common power supply line is set at a lowerlevel than that of the opposite electrode of the luminescent element.Since this potential of the common power supply line can be set lowenough different from the potential of the pixel electrode, the large“on” current in the second TFT can be obtained to get a high luminancedisplay. If the higher potential can be obtained in the second TFT whenthe pixel is turned “on”, the potential of the image signal can belowered to reduce its amplitude, and this results in a reduction of thedriving voltage in the display apparatus. Therefore, there areadvantages in reducing the power consumption, and simultaneously theproblem of withstanding voltage, which concerns each element formed by athin film, is not encountered.

In accordance with the present invention, if the second TFT is of an Nchannel type, it is preferable that the potential of the image signalsupplied through the data line to the pixel to be turned “on” state islower than, or is equal to, the potential of the opposite electrode. Inthis structure, the amplitude of the image signal can also be reduced toreduce the driving voltage in the display apparatus while keeping thesecond TFT in the “on” state.

In accordance with the present invention, if the second TFT is of an Nchannel type, it is preferable that the potential of the image signalsupplied through the data line to the pixel to be “off” state is higherthan, or is equal to, the potential of the common power supply line.That is, when the pixel is turned “off”, the gate voltage (the imagesignal) is not applied enough to turn the second TFT “off” completely.The “off” state can be realized, combined with non-linearcharacteristics of the luminescent element. Accordingly, the amplitudeof the image signal can be reduced to decrease the driving voltage inthe display apparatus and to increase the frequency of the image signal.

In accordance with the present invention, if the second TFT is of a Pchannel type, conversely to each of the above described structures, arelative relation of each potential is reversed. That is, when thesecond TFT is of a P channel type, a display apparatus is featured thatthe potential of common power supply line is set at a higher level thanthat of the opposite electrode. In this case, it is preferable that thepotential of the image signal supplied through the data line to thepixel to be “on” state is higher than, or is equal to, the potential ofthe opposite electrode. It is also preferable that the potential of theimage signal supplied through the data line to the pixel to be “off”state is lower than, or is equal to, the potential of the common powersupply line.

In accordance with the present invention, it is preferable that thefirst TFT and the second TFT are formed by TFTs which are in oppositeconduction types. That is, it is preferable that if the first TFT is ofan N channel type, the second TFT is of a P channel type, while if thefirst TFT is of a P channel type, the second TFT is of an N channeltype. In this structure, as will be described later in relation to Claim8 in detail, speeding up of the display operation can be achieved, byonly changing the potential of the image signal to turn “on” to thedirection that the resistance of the first TFT at the “on” state isreduced within the range of the driving voltage in the displayapparatus. Since it means that the potential of the image signal to putthe pixel “on” state is changed to the direction that the resistance ofthe second TFT at the “on state” is reduced at this time, as a result, adisplay luminance can be improved. Thus, reduction at the drivingvoltage and improvement in the display quality can be accomplishedsimultaneously.

Another embodiment of the invention describes a display apparatuscomprising, arranged on a substrate, a plurality of scanning lines, aplurality of data lines intersecting the scanning lines, a plurality ofcommon power supply lines, a plurality of pixels formed by the scanninglines and the data lines in a matrix form, each pixel comprising a firstTFT having a first gate electrode which is supplied with a scanningsignal through the scanning line, a holding capacitor which holds animage signal supplied through the data line via the first TFT, a secondTFT having a second gate electrode which is supplied with the imagesignal held by the holding capacitor, and a luminescent elementcomprising an emitting thin film which is provided between a pixelelectrode formed by each of the pixels and an opposite electrode opposedto the pixel electrode and emits light due to the driving current whichflows between the pixel electrode and the opposite electrode when thepixel electrode is electrically connected to the common power supplyline through the second TFT, wherein the first TFT and the second TFTare formed as the thin film transistors which are of opposite conductiontypes relative to each other.

In accordance with the present invention, since the first TFT and thesecond TFT are of opposite conduction types, for example, if the firstTFT is of an N channel type, the second TFT is of a P channel type, aheight of the selecting pulse is to be raised to increase a storagecapacity of the first TFT, while the potential of the image signal is tobe lowered to reduce the “on” resistance of the second TFT and toincrease an emitting luminance. These optimizations in the scanningsignal and the image signal are effective at shifting the gate voltageof the first TFT to the direction of increasing the “on” current of thisTFT in accordance with that of the image signals, which is at the levelto turn the luminescent element “on” state, are written into the holdingcapacitor, during the selection of the pixel. Therefore, the imagesignal can be written into the holding capacitor smoothly from the dataline through the first TFT. The gate voltage of the first TFT in theevent of selecting the pixel corresponds to the potential differencebetween the scanning signal at the higher potential side and thepotential-holding electrode at the time of “on”(the potential of theimage signal to turn “on”, the potential of the holding capacitor or thegate electrode potential of the second TFT). The gate voltage of thesecond TFT corresponds to the potential difference between thepotential-holding electrode at the time of “on” and the common powersupply line. When using the potential of the potential-holding electrodeat the time as a reference, the potential of the scanning signal at thehigher potential side and the common power supply line are the same inpolarity. If the potential of the potential-holding electrode at thetime of “on” (the potential of the image signal to turn “on”) ischanged, both gate voltages of the first TFT and the second TFT changecorrespondingly, in the same direction and by the same amount.Therefore, a speed up of the display operation can be accomplished,provided that the image signal potential to turn “on” is changed todecrease the resistance of the first TFT at the time of “on”. At thistime, since the potential of the image signal to turn “on” is changed inthe direction that the resistance of the first TFT at the time of “on”is reduced, as a result, a display luminance can be improved. Thus thereduction in the driving voltage and improvement in the display qualitycan be accomplished simultaneously.

In accordance with the present invention, it is preferable that the gatevoltage applied to the second TFT in the pixel at “off” state is in thesame polarity as of the second TFT in the “on” state, and also the valueof the gate voltage does not exceed the threshold voltage of the secondTFT (Claim 9). That is, when the pixel is turned “off”, the gate voltage(the image signal) is not applied enough to completely turn the secondTFT “on” state. Thus, the amplitude of the image signal can be reducedto achieve the increased frequency of the image signal.

In this structure, if the first TFT is of an N channel type and thesecond TFT is of a P channel type, it is preferable that the potentialsof the scanning signal to turn the first TFT “on” and the common powersupply line are the same, and the potential of the gate electrodeapplied to the second TFT of the pixel in the “off” state is lower thanthe potential which is obtained by subtracting the threshold voltage ofthe first TFT from the scanning signal potential at which the first TFTis turned “on”. In contrast, if the first TFT is of a P channel type andthe second TFT is of an N channel type, it is preferable that thepotential of the scanning signal when the first TFT is turned “on” isthe same as that of the common power supply line, and also the potentialof the gate electrode applied to the second TFT of the pixel in the“off” state is higher than the potential which is obtained by adding thethreshold voltage of the first TFT to the scanning signal potential atwhich the first TFT is turned “on”. As described above, if the potentialof the scanning signal when the first TFT is turned “on” and that of thecommon power supply line are equated, the number of levels of eachdriving signal is reduced. Thus, the number of input terminals to thedisplay apparatus and the number of power sources can be reducedsimultaneously, and this results in reduced power consumption.

In accordance with the present invention, it is preferable that one ofthe electrodes, which is provided at the holding capacitor and isopposite to the electrode to be electrically connected to the secondgate electrode of the second TFT, is supplied with a pulse, a potentialpolarity of which is opposite to the selecting pulse of the scanningsignal with a delay behind the selecting pulse. In this structure, sincethe writing of the image signals into the holding capacitor can besupplemented, the potential of the image signal applied to the gateelectrode of the second TFT can be shifted in the direction to increasea luminance, without increasing the amplitude of the image signal.

Further, another embodiment of the invention describes a displayapparatus comprising, arranged on a substrate, a plurality of scanninglines, a plurality of data lines intersecting the scanning lines, aplurality of common power supply lines, a plurality of pixels formed bythe scanning lines and the data lines in a matrix form, each pixelcomprising a first TFT having a first gate electrode which is suppliedwith a scanning signal through the scanning line, a holding capacitorwhich holds an image signal supplied through the data line via the firstTFT, a second TFT having a second gate electrode which is supplied withthe image signal held by the holding capacitor, and an luminescentelement comprising an emitting thin film which is provided between apixel electrode formed by each of the pixels and an opposite electrodeopposed to the pixel electrode and emits light due to the drivingcurrent which flows between the pixel electrode and the oppositeelectrode when the pixel electrode is electrically connected to thecommon power supply line through the second TFT, wherein one ofelectrodes of the holding capacitor, opposite to that electricallyconnected to the second gate electrode of the second TFT, is suppliedwith a pulse, the potential polarity of which is opposite to theselecting pulse of the scanning signal with a delay behind the selectingpulse.

In this structure, since the writing of the image signals into theholding capacitor can be supplemented, the potential of the image signalapplied to the gate electrode of the second TFT can be shifted in thedirection to increase a luminance, without increasing the amplitude ofthe image signal.

In any of the aforementioned embodiments, an organic semiconductor filmcan be used as the emitting thin films, for example.

In accordance with the present invention, in any of the aforementionedembodiments, the second TFT can be formed so as to perform in thesaturated region to prevent an abnormal current from generating in theluminescent element, which would result in the generation of across-talk, etc. at another pixel because of the voltage drop, or thelike.

Further, it is possible to prevent unevenness of the threshold voltagefrom influencing a display operation by forming the second TFT so as tooperating in the linear region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view schematically illustrating a display apparatus inaccordance with the present invention.

FIG. 2 is a block diagram illustrating a basic construction of a displayapparatus in accordance with the present invention.

FIG. 3 is an exploded plan view illustrating a pixel in the displayapparatus shown in the FIG. 2.

FIG. 4 is a cross-sectional view taken on line A-A′ of FIG. 3.

FIG. 5 is a cross-sectional view taken on line B-B′ of FIG. 3.

FIG. 6(A) is a cross-sectional view taken on line C-C′ of FIG. 3, andFIG. 6(B) is a schematic representation indicating an effect when theapparatus is constructed as shown in FIG. 6 (A).

FIG. 7(A) and FIG. 7(B) are cross-sectional views of the luminescentelements utilized in the display apparatus shown in FIG. 2,respectively.

FIG. 8(A) and FIG. 8(B) are cross-sectional views of the luminescentelements having a different structure from the pixels shown in FIG. 7(A)and 7(B), respectively.

FIG. 9 is a graph showing a current-voltage characteristic of theluminescent elements shown in FIG. 7(A) and FIG. 8(B).

FIG. 10 is a graph showing a current-voltage characteristic of theluminescent elements shown in FIG. 7(B) and FIG. 8(A).

FIG. 11 is a graph showing current-voltage characteristics of an Nchannel type of TFT.

FIG. 12 is a graph showing current-voltage characteristics of a Pchannel type of TFT.

FIG. 13(A)-FIG. 13(B) is a flow sectional view illustrating a method forproducing a display apparatus in accordance with the present invention.

FIGS. 14(A) and (B) are respectively a plan view and a cross-sectionalview of the pixels having a different structure from the pixels shown inFIGS. 3 through 6.

FIG. 15 is an equivalent circuit diagram illustrating a pixel structureof a display apparatus in accordance with Embodiment 1 of the presentinvention.

FIGS. 16(A) and (B) are a schematic representation indicating electricalconnections of each element formed in the pixel shown in FIG. 15, and awaveform chart indicating potential changes in the driving signal, etc.,respectively.

FIG. 17 is an equivalent circuit diagram illustrating a structure of adisplay apparatus in accordance with a modified Embodiment 1 of thepresent invention.

FIGS. 18(A) and (B) are a schematic representation indicating electricalconnections of each element formed in the pixel shown in FIG. 17, and awaveform chart indicating potential changes in the driving signal, etc.respectively.

FIG. 19 is an equivalent circuit diagram illustrating a pixel structureof a display apparatus in accordance with Embodiment 2 of the presentinvention.

FIGS. 20(A) and (B) are a schematic representation indicating electricalconnections of each element formed in the pixel shown in FIG. 19, and awaveform chart indicating potential changes in the driving signal, etc.respectively.

FIG. 21 is an equivalent circuit diagram illustrating a pixel structureof a display apparatus in accordance with a modified Embodiment 2 of thepresent invention.

FIGS. 22(A) and (B) are a schematic representation indicating electricalconnections of each element formed in the pixel shown in FIG. 21, and awaveform chart indicating potential changes in the driving signal, etc.respectively.

FIG. 23 is an equivalent circuit diagram illustrating a pixel structureof a display apparatus in accordance with Embodiment 3 of the presentinvention.

FIGS. 24(A) and (B) are a waveform chart indicating the signals to drivethe pixel shown in FIG. 23, and a schematic representation indicatingcorrespondences between these signals and the equivalent circuit,respectively.

FIG. 25 is a waveform chart indicating the signals to drive the pixel ofa display apparatus in accordance with Embodiment 2 of the presentinvention.

FIG. 26 is an equivalent circuit diagram illustrating a pixel structureof a display apparatus in accordance with a modified Embodiment 3 of thepresent invention.

FIGS. 27(A) and (B) are a waveform chart indicating the signals to drivethe pixel shown in FIG. 26, and a schematic representation indicatingcorrespondences between these signals and the equivalent circuit,respectively.

FIGS. 28(A) and (B) are an equivalent circuit diagram illustrating apixel of a display apparatus in accordance with Embodiment 4 of thepresent invention, and a waveform chart indicating the signals to drivethe pixel, respectively.

FIG. 29 is a block diagram of the driving circuit at the scanning sideto generate the signals shown in FIG. 28.

FIG. 30 is a waveform chart indicating each signal generated from thedriving circuit at the scanning side shown in FIG. 29.

FIG. 31 is a block diagram of a display apparatus.

FIG. 32 is an equivalent circuit diagram illustrating a conventionalpixel construction in the display apparatus shown in FIG. 31.

FIGS. 33(A) and (B) are a waveform chart indicating the signals to drivethe pixel shown in FIG. 32, and a schematic representation indicatingcorrespondences between these signals and the equivalent circuit,respectively.

FIGS. 34(A) and (B) are a block diagram indicating a construction toform a capacitor by using an adjacent gate line and a waveform of thegate voltage signals, respectively.

BEST MODE FOR CARRYING OUT OF THE INVENTION

Embodiments of the invention will be described with reference to thedrawings. Before showing each embodiment of the invention, referencewill be made to the common structure for each embodiment, wherein theportions having common functions in each embodiment are designated bythe same reference numerals, respectively, to avoid duplicatedescription.

(The General Construction of an Active Matrix Substrate)

FIG. 1 is a schematic block diagram schematically illustrating a generallayout of a display apparatus, and FIG. 2 is an equivalent circuitdiagram of an active matrix formed therein.

As illustrated in FIG. 1, the center portion of a transparent substrate10 as a basic body is a display portion 2, in a display apparatus 1 ofthe embodiment. In the peripheral region of the transparent substrate10, at the upper and lower sides as viewed on the figure, an inspectioncircuit 5, and a driving circuit 3 at the data side, which outputs animage signal to a data line “sig”, are formed respectively. And in theright and left sides as viewed on the figure, driving circuits 4 at thescanning side, which output scanning signals to a scanning line “gate”,are formed. In each of the driving circuits 3,4, a complementary typeTFT comprising a shift register circuit, a level shifter circuit, ananalogue switch circuit, etc. is formed by an N type TFT and a P typeTFT. A packaging pad 6, which is a terminal group for inputting theimage signals, the various electric potentials and the pulse signals, isformed in the peripheral region on the transparent substrate 10, outsideof the data side driving circuit 3.

In the display apparatus 1, a plurality of the scanning lines “gate” anda plurality of the data lines “sig”, which extend in a direction thatintersects the direction in which the scanning lines “gate” extend, areformed on the transparent substrate 10, in the same way as in an activematrix substrate of a liquid crystal display apparatus. As shown in FIG.2, many pixels 7 are formed in a matrix form by the crossing of the datalines “sig” and the scanning lines “gate”.

In any of the pixels 7, a first TFT 20 is formed, in which a scanningsignal is supplied to a gate electrode 21 (a first gate electrode)through the scanning line “gate”. One side of a source-drain region ofthe TFT 20 is electrically connected to the data line “sig”, while theother side of the source-drain region is electrically connected to apotential-holding electrode “st”. That is, a capacitor line “cline” isformed parallel to the scanning line “gate” and a holding capacitor“cap” is formed between the capacitor line “cline” and thepotential-holding electrode “st”. Accordingly, when the first TFT 20 isselected by the scanning signal and turned to “on”, an image signal,which is supplied from the data line “sig” and forwarded through thefirst TFT 20, is written in the holding capacitor “cap”.

A gate electrode 31 (a second gate electrode) of the second TFT 30 iselectrically connected to the potential-holding electrode “st”. Whileone side of the source-drain region of the second TFT 30 is connected toa common power supply line “com”, the other side of the source-drainregion is electrically connected to one of electrodes (a pixelelectrode, as will be referred to below) of an luminescent element 40.The common power supply line “com” is held at a constant potential. Whenthe second TFT 30 is turned “on”, the electric current from the commonpower supply line “com” flows to illuminate the luminescent element 40through the second TFT 30, and makes the luminescent element 40 emit.

In the display apparatus 1 constructed as stated above, since thedriving current flows through the current route formed by theluminescent element 40, the second TFT 30 and the common power supplyline “com”, the flow of the current stops when the second TFT 30 isturned “off”. However, in the display apparatus 1 according to thisembodiment, when the first TFT 20 is selected by the scanning signal andturned “on”, the image signal, which is supplied from the data line“sig” and forwarded through the first TFT, is written into the holdingcapacitor “cap”. Accordingly, since the potential of the gate electrodeof the second TFT 30 is held at equal to that of the image signal by theholding capacitor “cap”, even if the first TFT 20 is turned “off”, thesecond TFT 30 remains “on”. Therefore, the driving current in theluminescent element 40 keeps flowing and this pixel remains illuminated.This state will be maintained until new image data is written into theholding capacitor “cap” and the second TFT 30 turns “off”.

In the display apparatus 1, various arrangements are possible with thecommon power supply line “com”, the pixel 7 and the data line “sig”. Inthis embodiment, a plurality of the pixels 7, having the luminescentelements 40 power supply, in which the driving current is supplied viacommon power supply line “com”, are disposed on both sides of the commonpower supply line “com”. Two data lines “sig” are arranged at the sideof these pixels 7 opposite to the common power supply line “com”. Thatis, a unit including data line “sig”, a pixel group connected to thisdata line, a piece of common power supply line “com”, another pixelgroup connected to this common power supply line and another data line“sig” which supplies the pixel signals to this pixel group, is repeatedin the direction that the scanning line “gate” extends. Each commonpower supply line “com” supplies driving currents to two lines of thepixels 7. In this embodiment, in the each of two pixels 7 provided withthe common power supply line “com” therebetween, the first TFTs 20, thesecond TFTs 30 and the luminescent elements 40 are disposedsymmetrically with respect to the common power supply line “com” inorder to make the electrical connection between these elements and eachof wiring layers easier.

Thus, in this embodiment, since a piece of the common power supply line“com” drives two lines of the pixels, only one half of the number of thecommon power supply lines “com” is needed compared with the number ofthe common power supply lines “com” when each of them is formed per oneline of the pixels, and also the space reserved between the common powersupply line “com” and the data line “sig” formed in the same layer canbe eliminated. Therefore, the area for wiring can be reduced on thetransparent substrate 10, and consequently, the display performance canbe improved in terms of luminance, contrast ratio and so forth. Inaddition, since the common power supply lines are arranged such thateach common power supply line “com” is connected to two lines of thepixels, each two data lines “sig” are arranged side-by-side and supplythe image signal to each line of the pixels group.

(The Pixel Construction)

The structure of each pixel 7 in the display apparatus 1 formed asdescribed above will be explained in detail with reference to FIGS. 3 to6.

FIG. 3 is an enlarged plan view showing three pixels 7 of a plurality ofpixels 7 formed in the display apparatus 1 of this embodiment. FIG. 4,FIG. 5 and FIG. 6 are section views taken on line A-A′, B-B′ and C-C′ ofFIG. 3, respectively.

First, at the position taken on line A-A′ in FIG. 3, a silicon film 200,shaped like islands, is formed on the transparent substrate 10 per eachpixel 7 to form the first TFT 20, as shown in FIG. 4. A gate insulatingfilm 50 is formed on the surface of the silicon film 200. The gateelectrode 21 (a portion of the scanning line “gate”) is formed on thesurface of the gate-insulating film 50. The source-drain regions 22 and23 are formed by self-alignment with respect to the gate electrode 21.On the front side of the gate insulating film 50, a firstinner-layer-insulating film 51 is formed. The source drain-regions 22and 23 are electrically connected to the data line “sig” and thepotential-holding electrode “st”, respectively, through contact holes 61and 62 formed in the inner-layer-insulating film.

In each pixel 7, in parallel with the scanning line “gate”, thecapacitor line “cline” is formed in the same inner-layer as the scanningline “gate” and the gate electrode 21 (between the gate-insulating film50 and the first inner-layer-insulation film 51). An extension “st1” ofthe potential-holding electrode “st” overlays on this capacitor line“cline” through the first inner-layer-insulation thin film 51. In thisway, the capacitor line “cline” and the extension “st1” of the potentialholding electrode “st” form the holding capacitor “cap” incorporatingthe first inner-layer-insulation film 51 as a dielectric film. On thefront side of the potential-holding electrode “st” and on that of thedata line “sig”, a second inner-layer-insulation thin film 52 is formed.

At the position indicated by line B-B′ in FIG. 3, as shown in FIG. 5,two of the data lines “sig”, which correspond to each pixel 7, arearranged in parallel on the surfaces of the first and the secondinner-layer-insulation films 51 and 52 formed on the transparentsubstrate 10.

At the position indicated by line C-C′ in FIG. 3, as shown in FIG. 6(A), a silicon film 300, shaped like islands, is formed on thetransparent substrate 10 to form the second TFT 30, spreading across twopixels, which sandwich the common power supply line “com” therebetween.The gate insulating film 50 is formed on the surface of the silicon film300. The gate electrodes 31, corresponding to each pixel 7, are formedon the surface of the gate insulating film 50, sandwiching the commonpower supply line “com”, and the source-drain regions 32 and 33 areformed by self-alignment with respect to the gate electrodes 31. On thefront side of the gate insulating film 50, the firstinner-layer-insulation film 51 is formed. A source drain-region 62 iselectrically connected to a junction electrode 35 through a contact hole63 formed on the first inner-layer-insulation film 51. On the otherhand, the common power supply line “com” is electrically connected to aportion of the source-drain region 33, which is common between two ofthe pixels provided at the center of the silicon film 300, through acontact hole 64 of the first inner-layer-insulation film 51. On thesurfaces of the common power supply line “com” and the junctionelectrode 35, the second inner-layer-insulation film 52 is formed. Onthe surface of the second inner-layer-insulation film 52, a pixelelectrode 41 is formed that includes an ITO film. This pixel electrode41 is electrically connected to the junction electrode 35 through acontact hole 65 formed on the second inner-layer-insulation film 52, andthen electrically connected to the source drain region 32 of the secondTFT 30 through the junction electrode 35.

(Characteristics of the Luminescent Element)

Since any type of structures of the luminescent element 40 can be usedin the apparatus of the invention, a typical structure will be describedbelow.

First, the pixel electrode 41 comprising the ITO film constitutes oneelectrode (the positive electrode) of the luminescent element 40, asshown in FIG. 7 (A). On the surface of the pixel electrode 41, a holeinjection layer 42 and an organic semiconductor film 43, as an emittingthin film, are laminated. Further, an opposite electrode “op” (thenegative electrode) comprising a metal film, such as alithium-containing aluminum film or a calcium film, is formed on thesurface of the organic semiconductor film 43. This opposite electrode“op” is to be a common electrode formed entirely, or in stripedpatterns, on the transparent substrate 10, and is held at a constantpotential. In contrast, when the driving current flows in the reversedirection to the luminescent element 40 that is shown in FIG. 7 (A), theluminescent element 40 may be formed as shown in FIG. 7 (B). In thisstructure, the pixel electrode 41 (the negative electrode) comprisingthe ITO film, the lithium-containing aluminum electrode 45, which isvery thin to be almost transparent, the organic semiconductor layer 43,the hole injection layer 42, the ITO film layer 46 and the oppositeelectrode “op” (the positive electrode), comprising a metal film such asthe aluminum containing lithium film or the calcium film, are laminatedin this order from bottom to top layer. In this structure, even in thecase that the driving current of opposite polarity flows in each of theluminescent elements 40 shown in FIGS. 7 (A) and (B), the emittingcharacteristics of the elements 40 are not changed since the structureof the electrode layers, with which the hole injection layer 42 and theorganic semiconductor layer 43 contact directly, are the same structuresas the former. Any of the luminescent elements 40 shown in FIGS. 7 (A)and (B) has the pixel electrode 41 comprising the ITO film in theunder-layer side (the substrate side). Light is emitted from thebackside of the transparent substrate 10 through the pixel electrode 41and the transparent substrate 10, as shown by the arrow “hν”.

In contrast, when the luminescent element 40 is formed as shown in FIGS.8 (A) and (B), light is emitted from the front side of the transparentsubstrate 10 through the opposite electrode “op”, as shown by the arrow“hν”. That is, as shown in FIG. 8 (A), the organic semiconductor layer43 and the hole injection layer 42 are laminated on the surface of thepixel electrode 41 (the negative electrode) comprising a metal film,such as aluminum containing lithium. Further, the opposite electrode“op” comprising the ITO film (the positive electrode) is formed on thesurface of the hole injection layer 42. This opposite electrode “op” isalso a common electrode formed entirely, or in striped patterns, and isheld at a constant potential. In contrast, in order to flow the drivingcurrent in the reverse direction of the luminescent element that isshown in FIG. 8 (A), the luminescent element 40 may be formed as shownin FIG. 8 (B). This luminescent element 40 is formed by the pixelelectrode 41 (the positive electrode) comprising the metal thin filmsuch as aluminum containing lithium, the ITO film layer 46, the holeinjection layer 42, the organic semiconductor layer 43, thelithium-containing aluminum electrode 45, which is very thin to bealmost transparent, and the opposite electrode “op” (the negativeelectrode), comprising the ITO film, which are laminated from bottom totop in this order.

When forming any type of structures of the luminescent element 40, themanufacturing process is not complicated, even if the top-and-bottompositional relationship is reversed, provided that the hole injectionlayer 42 and the organic semiconductor layer 43 are formed inside of abank layer “bank” by an ink jet method as described below. Further, inthe case that the lithium-containing aluminum electrode 45, which isvery thin to be almost transparent, and the ITO film layer 46 are added,there is no obstacle to displaying images, even if thelithium-containing aluminum electrode 45 is laminated in the same regionof the pixel electrode 41 or if the ITO film 46 is laminated in the sameregion of the opposite electrode “op”. Therefore, the lithium-containingaluminum electrode 45 and the pixel electrode 41 can be patterned,either separately or simultaneously, using the same resist-mask.Similarly, the ITO film layer 46 and the opposite electrode “op” can bepatterned, either separately or simultaneously, using the sameresist-mask. The lithium-containing aluminum electrode 45 and the ITOfilm layer 46 may be formed only at the inside region of the bank layer“bank” as a matter of course.

Further, the opposite electrode “op” may be formed by the ITO film, andthe pixel electrode 41 may be formed by the metal film. In any case,light is emitted from the transparent ITO film.

The voltage is applied across the opposite electrode “op” as thepositive electrode, and the pixel electrode 41 as the negativeelectrode, of the luminescent element 40 formed as described above. Asshown in FIG. 9 (ampere-volt characteristics of the luminescent element40 shown in FIG. 7 (A) and FIG. 8(B)), and FIG. 10 (ampere-voltcharacteristics of the luminescent element 40 shown in FIG. 7 (B) andFIG. 8(A)), the current through the organic semiconductor layer 43 (thedriving current) increases suddenly in the region where the appliedvoltage (x-axis/the potential of the opposite electrode “op” to thepixel electrode 41) rises above the threshold value and there is“on-state”, i.e., the low resistance state. Consequently, theluminescent element 40 emits light as the electro-luminescence elementor as the LED element. This emitting light from the luminescent element40 is reflected by the opposite electrode “op” and is emitted throughthe transparent pixel electrode 41 and the transparent substrate 10. Incontrast, in the region where the applied voltage (x-axis/the potentialof the opposite electrode “op” to the pixel electrode 41) drops belowthe threshold voltage, the “off-state”, i.e., the high resistance stateis provided, and the current through the organic semiconductor layer 43(the driving current) stops. Consequently the light luminescent element40 is turned “off”. The threshold voltages in the examples shown inFIGS. 9 and 10 are approximately +2 V and approximately −2 V,respectively.

Although the light emitting efficiency tends to somewhat decline, thehole injection layer 42 may be omitted. There may be a case that withoutincorporating the hole injection layer 42, an electron injection layeris formed at the opposite position to where the hole injection layer 42is formed with respect to the organic semiconductor layer 43. Further,both the hole injection layer 42 and the electron injection layer may beincorporated.

(TFT Characteristics)

As the TFTs (the first TFT 20 and the second TFT 30 shown in FIG. 2) forcontrolling light emission of the luminescent element 40 that is formedas described above, the ampere-volt characteristics of N channel typeand P channel type TFTs are shown in FIGS. 11 and 12, respectively (inany of the Figures, examples of the drain voltages are 4V and 8V areshown). As understood by these Figures, the TFT operates “ON-OFF”control action depending on the gate voltage applied across the gateelectrode. That is, when the gate voltage rises over the thresholdvoltage, the TFT will be in “on-state” (the low resistance state) toincrease the drain current. In contrast, when the gate voltage decreasesbelow the threshold voltage, the TFT will be “off-state” (the highresistance state) to reduce the drain current.

(A Method of Producing the Display Apparatus)

In a method of producing the display apparatus 1 that is formed asdescribed above, the steps up to the formation of the first TFT 20 andthe second TFT 30 on the transparent substrate 10 are almost the same asthe steps for producing the active matrix substrate of the liquidcrystal display apparatus 1. Accordingly, a general description will bemade simply with reference to FIG. 13.

FIG. 13 is a diagrammatic flow sectional view illustrating the steps forforming each component of the display apparatus 1 under a temperaturecondition below 600° C.

As shown in FIG. 13 (A), a groundwork protection film (not shown infigure) comprising a silicon oxide film ranging about 2000 to 5000 Å inthickness is formed, as needed, on the transparent substrate 10 by aplasma enhanced CVD method utilizing TEOS (tetraethoxysilane) or oxygengas, etc., as a material. Then, after the substrate temperature is setat 350° C., a semiconductor film 100 comprising an amorphous siliconfilm ranging about 300 to 700 Å thick is formed on the surface of thegroundwork protection film by a plasma enhanced CVD method. Next, thesemiconductor film 100 comprising the amorphous silicon film issubjected to crystallization such as laser annealing or a solid-phasegrowth method to crystallize the semiconductor film 100 into apoly-silicon film. The laser annealing utilizes, for example, an excimerlaser line beam having a long side of 400 mm, and its output power is,for example, 200 mJ/cm². The line beams are scanned so that the linebeams overlap with each other at portions corresponding to 90% of thepeak laser power in the short side.

Then, as shown in FIG. 13 (B), by a patterning, the semiconductor film100 is formed into semiconductor films 200, 300, shaped as islands, andon the surface of them, the gate insulating film 50 comprising a siliconoxide film or a silicon nitride film, ranging about 600 to 1500 Å inthickness, is formed by a plasma enhanced CVD method utilizing TEOS(tetraethoxysilane) or oxygen gas, etc., as a material.

Next, as shown in FIG. 13 (C), after a conductive film comprising ametal film, such as aluminum, tantalum, molybdenum, titanium, tungsten,etc., is formed by a sputtering method, the gate electrodes 21 and 31,as portions of the scanning lines “gate”, are formed by a patterning. Inthis step, the capacitor line “cline” is also formed. In the figure,reference number 310 indicates an extensional part of the gate electrode31.

At this state, by an implantation of impurities, such as highconcentration phosphorus ions or boron ions, etc., the source-drainregions 22, 23, 32, and 33 are formed by self-alignment with respect tothe gate electrodes 21 and 31 on the silicon films 200 and 300. Theportions where the impurity is not implanted are channel regions 27 and37. In this embodiment, a different conduction type TFT may be formed onthe same substrate, as will be described later. In this case, in theimpurity implantation step, the impurity implantation will be performedmasking a region to form the opposite conduction type TFT.

Then, as shown in FIG. 13 (D), after the inner-layer-insulation film 51is formed, the contact holes 61,62,63,64 and 69 are formed, and then thedata line “sig”, the potential-holding electrode “st” having theextended portion “st1” overlapped with the capacitor line “cline” andwith the extended portion 310 of the gate electrode 31, the common powersupply line “com”, and the junction electrode 35 are formed.Consequently, the potential-holding electrode “st” is electricallyconnected to the gate electrode 31 through the contact hole 69 and theextended portion 310. As mentioned above, the first TFT 20 and thesecond TFT 30 are formed. Further, the holding capacitor “cap” is formedby the capacitor line “cline” and the extended portion “st1” of thepotential-holding electrode “st”.

Next as shown in FIG. 13 (E), the second inner-layer-insulation film 52is formed, and the contact hole 65 is formed at the place correspondingto the junction electrode 35 in this inner-layer insulation film. Then,after the conductive film is formed all over the surface of the secondinner-layer insulation film 52, patterning is performed, and the pixelelectrode 41 is formed to electrically connect the conductive film tothe source-drain region 32 of the second TFT 30 through the contact hole65.

Next as shown in FIG. 13 (F), after a black resist layer is formed onthe front side of the second inner-layer insulation film 52, a banklayer “bank” is formed, leaving this resist to surround the regions forforming the organic semiconductor film 43 of the luminescent element 40and the hole injection layer 42. In either case, where the organicsemiconductor film 43 is formed in a box shape independently per pixel,or formed in a stripe shape along the data line “sig”, this producingprocess in accordance with this embodiment can be applied only byforming the bank layer “bank” in a shape adapted thereto.

Liquid material (a precursor), for forming the organic semiconductorfilm 43, is injected into an inner region of the bank layer “bank” froman inkjet-head “IJ” to form the organic semiconductor film 43 in theinner region of the bank layer “bank”. Similarly, liquid material (aprecursor) for forming the hole injection layer 42 is injected into aninner region of the bank layer “bank” from the inkjet-head “IJ” to formthe hole injection layer 42. As seen from the description presentedabove concerning the construction of the luminescent element withreference to FIGS. 7 (A) and (B) and FIGS. 8 (A) and (B), an order ofsteps to form the organic semiconductor film 43 and the hole injectionlayer 42 may be interchangeable depending on the structure.

Since the bank layer “bank” comprises the resist, the layer is waterrepellent. In contrast, since the precursors of the organicsemiconductor film 43 and the hole injection layer 42 utilize ahydrophilic solvent, the coating region of the organic semiconductorfilm 43 is strictly defined by the bank layer “bank”, and the regioncannot extend off to an adjacent pixel. When the bank layer “bank” isformed at a sufficient height, the organic semiconductor film 43 or thehole injection layer 42 can be formed within a predetermined region by acoating method, such as a spin coating method, even if the ink jetmethod is not employed.

In this embodiment, in order to improve the production efficiency informing the organic semiconductor film 43 or the hole injection layer 42by the ink jet method, as shown in FIG. 3, the forming regions of theorganic semiconductor films 43 have the same inter-center pitch Pbetween adjacent pixels 7 lying along the extending direction of thescanning line “gate”. Therefore, as indicated in the arrow “Q”, there isan advantage that the material of the organic semiconductor film 43,etc. can be injected by an ink jet head “IJ” simply with the same pitchalong the extending direction of the scanning line “gate”. The samepitch injection also simplifies a device for transferring the ink jethead “IJ” while facilitating the improvement of injection accuracy.

Afterward, as shown in FIG. 13 (G), the opposite electrode “op” isformed on the front side of the transparent substrate 10. The oppositeelectrode “op” may be formed either on the entire surface or in astriped shape. In the latter, the patterning will be performed after thefilm is formed on the entire front side of the transparent substrate 10,and then patterning it into the striped shape.

The TFTs are also formed in the driving circuit at the data side 3 orthe driving circuit at the scanning side 4, as shown in FIG. 1. Thisforming process of the TFTs employs all or a part of the steps for theTFT formation in the above described pixel 7. Therefore, the TFTs of thedriving circuit are provided in the same inner-layer that the TFTs ofthe pixel 7 are formed in.

In this embodiment, since the bank layer “bank” comprises a black andinsulating resist, the resist is left as it is to be utilized as a blackmatrix “BM” and an insulating layer for reducing a parasiticcapacitance.

As shown in FIG. 1, the bank layer “bank” is also formed in theperipheral region of the transparent substrate 10 (hatched area in thefigure). Hence, as the driving circuit at the data side 3 as well as thedriving circuit at the scanning side 4 is overlaid by the bank layer“bank”, the bank layer “bank” is disposed between the wiring layer ofthe driving circuit and the opposite electrode “op”, even if theopposite electrode “op” and the forming regions of these drivingcircuits are overlapped. Therefore, the prevention of the drivingcircuits 3,4 from the parasitic capacitance can be achieved so as toreduce the load of the driving circuit at the data side 3, resulting inproviding reduced electric consumption or speeding up the displayoperation.

In this embodiment, as shown in FIGS. 3 through 5, the bank layer “bank”is formed so as to overlap with the data line “sig”. Thus, the banklayer “bank” is disposed between the data line “sig” and the oppositeelectrode “op”, and consequently, it is possible to prevent theparasitic capacitance in the data line “sig”. This results in thereduction of the load of the driving circuit, providing a reduction ofthe electric consumption or a speeding up of the display operation.

Further, in this embodiment, as shown in FIG. 3, FIG. 4 and FIG. 6(A),the bank layer “bank” is preferably also formed in the region where thepixel electrode 41 and the junction electrode 35 overlap. That is, asshown in FIG. 6(B), if the bank layer “bank” is not formed at the regionwhere the pixel electrode and the junction electrode 35 overlap, evenwhen the organic semiconductor film 43 emits light by the drivingcurrent across the pixel electrode 51 and the opposite electrode op, thelight cannot be emitted and does not contribute to the displayoperation. This is because the light gets in between the junctionelectrode 35 and the opposite electrode “op”. The driving currentequivalent to the light, that does not contribute to the displayoperation, may be called an ineffective current with respect to display.In this embodiment, however, since the bank layer “bank” is formed inthe region where the ineffective current is to flow to prevent theineffective current, the waste current in the common power supply line“com” can be prevented. Hence, the width of the common power supply line“com” can be reduced accordingly.

As discussed above, if the bank layer “bank” that includes a blackresist is reserved, the bank layer “bank” works as a black matrix toimprove the display image quality, such as a luminance and a contrastratio. That is, in the display apparatus according to this embodiment,since the opposite electrode “op” is formed in a striped shape on theentire surface, or on a broad region, of the front side of thetransparent substrate 10, reflected light from the opposite electrode“op” reduces the contrast ratio. However, in this embodiment, since thebank layer “bank”, which prevents the parasitic capacitance, includes ablack resist, with the forming region of the organic semiconductor film43 defined, there is an advantage that the bank layer “bank”, workingalso as the black matrix, blocks useless light reflected from theopposite electrode “op”, and it results in increasing the contrastratio. Further, since the emitting region can be defined byself-alignment utilizing the bank layer “bank”, a margin for alignmentrequired for the emitting region is not necessary. This margin has beenthe problem when another metal layer, etc., is used as the black matrixinstead of the bank layer “bank”.

(Another Structure of the Active Matrix Substrate)

The present invention can be applied to various types of active matrixsubstrates as well as the above described structure. For example, theinvention can be applied to the display apparatus 1A, wherein, asdescribed in reference to FIG. 31, a unit comprising a data line “sig”,a common power supply line “com” and a line of the pixels 7 is repeatedin the direction of the scanning line “gate” on a transparent substrate1.

The holding capacitor “cap” may be formed between the common powersupply line “com” and the potential-holding electrode “st” without thecapacitor line. In this case, as shown in FIGS. 14 (A) and (B), anextended portion 310 of the gate electrode 31 to connect electricallythe potential-holding electrode “st” and the gate electrode 31, isexpanded to the under-layer of the common power supply line “com” toform the holding capacitor “cap”. This holding capacitor “cap” has thefirst inner-layer-insulation film 51 which is located between theextended portion 310 and the common power supply line “com” for adielectric film.

As for the holding capacitor “cap” further, Figure is abbreviated, itmay be formed utilizing a poly-silicon film for forming the TFT, and italso may be formed with the ahead scanning line other than the capacitorline and the common power supply line.

Embodiment 1 for Carrying Out the Invention

FIG. 15 is an equivalent circuit diagram showing the pixel structure inthe display apparatus 1 in this embodiment. FIGS. 16 (A) and (B) are aschematic representation showing electrical connections of each elementformed in each pixel, and a wave form chart showing potential changes ofdriving signals and the like respectively.

In this embodiment, as shown in FIG. 15 and FIGS. 16 (A) and (B), thefirst TFT 20 is of an N channel type. Accordingly, when the potential ofthe scanning signal “S gate” supplied from the scanning line “gate”becomes high, the first TFT 20 turns “on”, and the image signal “data”is written into the holding capacitor “cap” from the data line “sig”through the first TFT 20. On the other hand, the potential of thescanning signal “Sgate” supplied from the scanning line “gate” is low,the second TFT is driven and controlled by the image signal “data” heldby the holding capacitor “cap”.

In this embodiment, the second TFT 30 is also of an N channel type.Therefore, from the data line “sig”, the image signal “data” in thehigher potential side is written into the holding capacitor “cap” of thepixel to be “on”, while the image signal “data” in the lower potentialside is written into the holding capacitor “cap” of the pixel to be“off”. The electric potential of the potential-holding electrode “st”varies in response to this.

The gate voltage “V gcur” of the second TFT 30 corresponds to thepotential difference between the potential-holding electrode “st” andthe lower one of the common power supply line “com” and the pixelelectrode 30. In this embodiment, the potential of the common powersupply line “com” is maintained lower than the potential of the oppositeelectrode “op” of the luminescent element 40, such that when the secondTFT 30 becomes “on”, the current flows from the luminescent element 40to the common power supply line “com”, as shown by the arrow “F”.Therefore, the gate voltage “V gcur” of the second TFT 30 corresponds tothe potential difference between the common power supply line “com” andthe potential-holding electrode “st”. Contrarily to the potential of thepixel electrode 30, which corresponds to an intermediate potentialbetween the common power supply line “com” and the opposite electrode“op”, the potential of the common power supply line “com” can be set lowenough. Therefore, in this embodiment, the gate voltage “V gcur” of thesecond TFT 30 can be maintained high enough and the “on” current of thesecond TFT 30 flows enough so that a display in a high luminance can beperformed. If the gate voltage “V gcur” of the second TFT 30 can be highenough when the pixel is turned “on”, the potential of thepotential-holding electrode “st”, i.e., the higher side potential of theimage signals “data” can be lowered correspondingly. Accordingly, theamplitude of the image signals “data” can be reduced to decrease thedriving voltage in the display apparatus 1.

In addition, although the “on” current of the second TFT 30 depends notonly on the gate voltage “V gcur” but also on the drain voltage, theaforementioned conclusion does not change.

In this embodiment, the “on” current of the second TFT 30 is defined bythe potential difference between the common power supply line “com” andthe holding electrode “st”, and is not affected by the potential of theopposite electrode “op” directly. Therefore, the higher side potentialof the image signal “data” to turn the pixel “on” is lowered below thepotential of the opposite electrode “op” to reduce the amplitude of theimage signal “data”, providing the reduced driving voltage in thedisplay apparatus 1. The higher side potential of the image signal“data” to turn the pixel “on” may be lowered to the same potential ofthe opposite electrode “op” to reduce the amplitude of the image signal“data”.

Further, in this embodiment, the potential of the image signal “data”,which is supplied from the data line “sig” to the pixels to be turned“off”, is set rather higher than the potential of the common powersupply line “com”. Since the second TFT 30 is of an N type, the gatevoltage “V gcur” of the second TFT 30 is required to be negative (alower potential than the common power supply line “com”) to make it turn“off” completely. Or, the lower side potential of the image signal“data” is set rather high such that an absolute value of the gatevoltage “V gcur” of the second TFT 30 becomes rather lower than anabsolute value of a threshold voltage of the second TFT 30. At thistime, in the pixel 7 which is in “off” state, the gate voltage of thesecond TFT 30 is set at the same polarity as the second TFT 30 which isturned “on”, and is also set lower than the threshold voltage of thesecond TFT 30. At this time, even if the lower side potential of theimage signal “data” is set rather high as discussed above, the flow of“on” current of the second TFT 30, being at the high resistance, is sosmall that the luminescent element 40 is kept “off”. The potential ofthe image signal “data” supplied from the data line “sig” to the pixelto be “off” may be set the same as the potential of the common powersupply line “com” to reduce the amplitude of the image signal “data”.

Thus, when the lower side potential of the image signal “data” is setrather high, but in the order of not exceeding the threshold value, theamplitude of the image signal “data” can be reduced so that the drivingvoltage of the image signal “data” can be reduced. Further, as thehigher side potential of the image signal “data” to turn the pixel “on”has been lowered below the potential of the opposite electrode “op”, asmentioned above, the potential of the image signal “data” falls withinthe range defined by the opposite electrode “op” and the common powersupply line “com”. Therefore, the driving voltage in the displayapparatus 1 can be reduced, and this results in lower electric powerconsumption in the display apparatus 1. This structure does not invitedeterioration in the image quality, abnormality in performance andreduction of frequency enabling operation. There is also an advantagethat a problem of withstanding voltage (the insulation resistance),which concerns each element formed by a thin film, is not encounteredbecause of the reduced driving voltage in the display apparatus 1.

A Modified Embodiment of Embodiment 1

FIG. 17 is an equivalent circuit diagram showing the pixel structure inthe display apparatus 1 of this embodiment. FIGS. 18 (A) and (B) are aschematic representation showing electrical connections of each elementformed in each pixel, and a wave form chart showing potential changes ofthe driving signals and like, respectively. In this embodiment, both thefirst and second TFT are formed with a P channel type, which isdifferent from Embodiment 1. In this embodiment, however, each elementis driven and controlled with the same technical idea as Embodiment 1and the structure is same as Embodiment 1, except that the polarities ofthe driving signals described in Embodiment 1 are reversed. Therefore,the structure will be simply described.

As shown in FIG. 17 and FIGS. 18 (A) and (B), since the first TFT 20 isof a P channel type in this embodiment, the first TFT 20 is “on” whenthe potential of the scanning signal supplied from the scanning line“Sgate” becomes low.

The second TFT 30 is also of a P channel type in this embodiment. Hence,from the data line “sig”, the image signal “data” at the lower sidepotential is written into the holding capacitor “cap” of the pixel to be“on” state, while the image signal “data” at the higher side potentialis written into the holding capacitor “cap” of the pixel to be “off”state.

The gate voltage “V gcur” at the second TFT 30 corresponds to thepotential difference between the potential-holding electrode “st” andthe higher one of the common power supply line “com” and the pixelelectrode 30. In this embodiment, the potential of the common powersupply line “com” is maintained higher than the potential of theopposite electrode “op” of the luminescent element 40, such that if thesecond TFT 30 becomes “on” state, the current flows from the commonpower supply line “com” to the luminescent element 40, as shown by thearrow “E”. Therefore, the gate voltage “V gcur” of the second TFT 30corresponds to the potential difference between the common power supplyline “com” and the potential-holding electrode “st”, The potential ofthe common power supply line “com” can be set high enough, differentfrom the potential of the pixel electrode 30 which corresponds to anintermediate potential between the common power supply line “com” andthe opposite electrode “op”. Therefore, in this embodiment, the gatevoltage “V gcur” of the second TFT 30 can be high enough so that the“on” current of the second TFT 30 is great, enabling a display in a highluminance. If the gate voltage “V gcur” of the second TFT 30 is highenough when the pixel is turned “on” state, the potential of thepotential holding electrode “st”, i.e., the lower side potential of theimage signal “data” can be raised correspondingly such that theamplitude of the image signal “data” can be reduced.

In this embodiment, since the “on” current of the second TFT 30 is notaffected by the potential of the opposite electrode “op” directly, thelower side potential of the image signal “data” to turn the pixel “on”is set rather higher than the potential of the opposite electrode “op”to reduce the amplitude of the image signal “data”. In addition, thelower side potential of the image signal “data” to turn the pixel “on”may be raised to the same potential as the opposite electrode “op” toreduce the amplitude of the image signal “data”.

Further, in this embodiment, the potential of the image signal “data”supplied from the data line “sig” to the pixel to be turned “off” is setrather lower than the potential of the common power supply line “com”.This means that the higher side potential of the image signal “data” isset rather low, such that an absolute value of the gate voltage “V gcur”of the second TFT 30 becomes rather lower than an absolute value of thethreshold voltage of this TFT. Therefore, the “on” current of the secondTFT 30 becomes so small that the luminescent element 40 is turned “off”.In addition, the potential of the image signal “data” supplied from thedata line “sig” to the pixel to be turned “off” may be set at the samepotential as the potential of the common power supply line “com” toreduce the amplitude of the image signal “data”.

Thus, since the lower side potential of the image signal “data” is setrather high, and the higher side potential of the image signal “data” toturn the pixel “on” is set rather low, the potential of the image signal“data” falls within the range defined by the opposite electrode “op” andthe common power supply line “com”. Therefore, it is possible to obtainthe same effects as Embodiment 1, such that the driving voltage in thedisplay apparatus 1 can be reduced, enabling the electric powerconsumption to be reduced in the display apparatus 1, and so on.

Embodiment 2 for Carrying Out the Invention

FIG. 19 is an equivalent circuit diagram showing the pixel structure inthe display apparatus 1 of this embodiment. FIGS. 20 (A) and (B) are aschematic representation showing electrical connections of each elementformed in each pixel, and a wave form chart showing potential changes inthe driving signal and the like, respectively. In this embodiment, asshown in FIG. 19 and FIGS. 20 (A) and (B), the first TFT is formed withan N channel type and the second TFT is formed with a P channel type.Since the second TFT 20 is of a P channel type, from the data line“sig”, the image signal “data” in the lower potential side is writteninto the holding capacitor “cap” of the pixel to be turned “on”. Theimage signal “data” in the higher potential side is written into theholding capacitor “cap” of the pixel to be turned “off”. The gatevoltage “V gcur” of the second TFT 30 corresponds to the potentialdifference between the potential-holding electrode “st” and the higherone of the common power supply line “com” and the pixel electrode 30.

In this embodiment, the potential of the common power supply line “com”is set higher than the potential of the opposite electrode “op” of theluminescent element 40, such that the gate voltage “V gcur” of thesecond TFT 30 corresponds to the potential difference between the commonpower supply line “com” and the potential-holding electrode “st”. Thepotential of the common power supply line “com” can be set higher enoughcompared with the potential of the pixel electrode 41, such that the“on” current of the second TFT 30 flows enough to achieve the display ina high luminance. And correspondingly, as the potential of thepotential-holding electrode “st”, i.e., the lower side potential of theimage signal “data” can be raised, the amplitude of the image signal“data” can be reduced. In addition, as the “on” current of the secondTFT 30 is not affected by the potential of the opposite electrode “op”directly, the lower side potential of the image signal “data” to turnthe pixel “on” is raised higher than, or equal to, the potential of theopposite electrode “op” to reduce the amplitude of the image signal“data”. Further, in this embodiment, the potential of the image signal“data” supplied from the data line “sig” to the pixel to be turned “off”is set rather lower than, or equal to, the potential of the common powersupply line “com” to reduce the amplitude of the image signal “data”.Therefore the potential of the image signal “data” falls within therange defined by the opposite electrode “op” and the common power supplyline “com”, and as a result the driving voltage in the display apparatus1 is reduced. This results in lower power consumption, etc., in thedisplay apparatus 1 the same as Embodiment 1 or modified Embodiment 1.

In this embodiment, since the first TFT 20 is of an N channel type,which is the opposite conductivity of the second TFT 30, the scanningline “gate” (the scanning signal “Sgate”), at the time of selecting thepixel, is at the higher potential. The gate voltage “Vgsw” of the firstTFT 20 at this time corresponds to the potential difference between thescanning signal “Sgate” at the higher potential and thepotential-holding electrode (the potential of the holding capacitor“st”, the potential of the gate electrode of the second TFT 30). Sincethe second TFT 30 is of a P channel type, the image signal “data” toturn the pixel “on” is at the lower potential side and the potential ofthe potential-holding electrode “st” is decreasing during the selectingperiod of the pixel 7. Therefore, the gate voltage “Vgsw” of the firstTFT 20 shifts toward increasing the “on” current.

On the other hand, the gate voltage “Vgcur” of the second TFT 30corresponds to the potential difference between the common power supplyline “com” and the potential-holding electrode “st”. As the potential ofthe potential-holding electrode “st” tends to decrease during theselecting period when the selected pixel 7 is turned “on”, the gatevoltage “Vgcur” of the second TFT 30 shifts toward increasing the “on”current.

As mentioned above, in this embodiment, since the first TFT 20 is theopposite conduction type to the second TFT 30, the selectingpulse-height of the scanning signal “Sgate” is raised to increase awriting capacity of the first TFT 20, while the image signal “data” isdecreased in order to reduce the “on” resistance of the second TFT 30,so as to increase a luminance of the luminescent element 40. Theseoptimizations of selecting pulse-height of the scanning signal “Sgate”and the image signal “data” are effective at shifting the gate voltageof the first TFT 20 toward increasing the “on” current of the first TFT20, in accordance with the image signals “data”, which are at the levelto turn the luminescent element 40 “on”, are written into the holdingcapacitor “cap” during the selecting period of the pixel 7. Therefore,the image signal “data” from the data line “sig” is smoothly writteninto the holding capacitor “cap” through the first TFT 20. The gatevoltage “Vgsw” of the first TFT 20 at the time of selecting the pixelelement 7 corresponds to the potential difference between the scanningsignal “Sgate” at the higher potential and the potential-holdingelectrode “st”(the potential of the holding capacitor “cap” or thepotential of the gate electrode of the second TFT 30). The gate voltage“Vgcur” of the second TFT 30 corresponds to the potential differencebetween the common power supply line “com” and the potential-holdingelectrode “st”. The potential of the scanning signal “Sgate” at thehigher side and the potential of the common power supply line “com” arethe same in polarity when using the potential of the potential-holdingelectrode “st” as a reference. Accordingly, if the potential of thepotential-holding electrode “st” is changed, both the gate voltage“Vgsw” of the first TFT 20 and the “Vgcur” of the second TFT 30correspondingly shift by the same amount in the same direction.Therefore, if the potential of the image signal “data” to turn “on” ischanged toward reducing the “on” resistance of the first TFT 20 withinthe range of the driving voltage of the display apparatus 1, the higherdisplay operation speed can be offered. At this time, since thepotential of the image signal “data” to turn “on” is changed in thedirection that the “on” resistance of the second TFT 30 is decreasing asa result, the luminance can be improved as well. This provides reduceddriving voltage and improved quality of the display, simultaneously.

A Modified Embodiment of Embodiment 2

FIG. 21 is an equivalent circuit diagram showing the pixel structure inthe display apparatus 1 of this embodiment. FIGS. 22 (A) and (B) are aschematic representation showing electrical connections of each elementformed in each pixel, and a wave form chart showing potential changes ofthe driving signal, and the like, respectively. In this embodiment, incontrast with Embodiment 2, the first TFT 20 is formed with a P channeltype and the second TFT 30 is formed with an N channel type. However, inthis embodiment, each element is driven and controlled with the sametechnical idea as Embodiment 2, except that the polarity of the drivingsignal described in Embodiment 2 is inverted. Accordingly, the structurewill be simply described.

As shown in FIG. 21 and FIGS. 22 (A) and (B), as in this embodiment, thesecond TFT 30 is of an N channel type like in Embodiment 1. From thedata line “sig”, the image signal “data” in the higher potential side iswritten into the holding capacitor “cap” of the pixel to be turned “on”,while the image signal “data” in the lower potential side is writteninto the holding capacitor “cap” of the pixel to be turned “off”. Thegate voltage “V gcur” of the second TFT 30 corresponds to the potentialdifference between the potential-holding electrode “st” and the lowerone of the common power supply line “com” and the pixel electrode 30. Inthis embodiment, since the potential of the common power supply line“com” is set lower than that of the opposite electrode “op” of theluminescent element 40, the gate voltage “V gcur” of the second TFT 30corresponds to the potential difference between the common power supplyline “com” and the potential-holding electrode “st”. The potential ofthe common power supply line “com” can be set low enough that the “on”current of the second TFT 30 flows enough to achieve the display in ahigh luminance. The potential of the potential-holding electrode “st”,i.e., the higher side potential of the image signal “data” can be raisedby the increment of the luminance to reduce the amplitude of the imagesignal “data”. And, since the “on” current of the second TFT 30 is notaffected by the potential of the opposite electrode “op” directly, thehigher side potential of the image signal “data” to turn the pixel “on”is set lower than, or equal to, that of the opposite electrode “op” soas to reduce the amplitude of the image signal “data”. Further, in thisembodiment, the potential of the image signal “data” supplied from thedata line “sig” to the pixel to be “off” is set rather higher than, orequal to, that of the common power supply line “com” to reduce theamplitude of the image signal “data”. Therefore, the potential of theimage signal “data” falls within the range defined by the oppositeelectrode “op” and the common power supply line “com” to reduce thedriving voltage in the display apparatus 1. This results in reducing thepower consumption, etc., in the display apparatus 1, just as effectivelyas Embodiment 1 or modified Embodiment 1.

In this embodiment, as the first TFT 20 is of a P channel type, which isthe opposite conductivity to the second TFT 30, the scanning line “gate”(the scanning signal “Sgate”), at the time of selecting the pixel, is atthe lower potential. In contrast, as the second TFT 30 is of an Nchannel type, the image signal “data” to turn the pixel “on” is at thehigher potential side.

As mentioned above, in this embodiment, since the first TFT 20 is of anopposite conductivity type to the second TFT 30, the potential of theselecting pulse of the scanning signal “Sgate” is set lower to increasethe writing capacity of the first TFT 20. The potential of the imagesignal “data” is decreased to increase a luminance of the luminescentelement 40 by reducing the “on” resistance of the second TFT 30. Theseoptimizations, of the selecting pulse-height of the scanning signal“Sgate” and of the image signal “data”, are effective at shifting thegate voltage of the TFT 20 toward increasing the “on” current of thefirst TFT 20, in accordance with the image signals “data” at the levelto turn “on” the luminescent element 40 are writing into the holdingcapacitor “cap”, during the period of selecting the pixel 7.Accordingly, since the potential of the scanning signal “Sgate” at thelower side and the potential of the common power supply line “com” arethe same in polarity when using the potential of the potential-holdingelectrode “st” as a reference, if the potential of the potential-holdingelectrode “st” is changed, the gate voltage “Vgsw” of the first TFT 20and the gate voltage “Vgcur” of the second TFT 30 correspondingly shiftby the same amount and in the same direction. Therefore, if thepotential of the image signal “data” to turn “on” is changed towardreducing the “on” resistance of the first TFT 20 within the range of thedriving voltage of the display apparatus 1, the display operation speedcan be increased. Since the potential of the image signal “data” to turn“on” is changed toward reducing the “on” resistance of the second TFT 30at the same time, a luminance can also be improved. This reduces drivingvoltage and improves the quality of the display simultaneously, just asin Embodiment 2. An optimized driving method in the above describedEmbodiment 2 and modified Embodiment 2 will be described with referenceto FIG. 25.

In Embodiment 2, the first TFT is of an N channel type and the secondTFT is of a P channel type. As shown in FIG. 25, when the luminescentelement 40 is turned “off”, the potential of the image signal “data” israised higher than the potential of the common power supply line “com”to turn “off” the P channel type second TFT 30. However, in thisembodiment, even when the luminescent element 40 is turned “off”, thesecond TFT 30 is not completely turned “off”, as shown in FIG. 25. Thatis, since the second TFT is of a P channel type in this embodiment, inorder to turn “off” the TFT completely, the gate voltage “Vgcur” isrequired to be 0 (the same potential as the potential of the commonpower supply line “com”) or positive (higher than the common powersupply line “com”). However, in this embodiment, the potential of theimage signal “data” when the luminescent element is turned “off” is setat rather lower so that the gate voltage “Vgcur” of the second TFT 30becomes rather higher than the threshold voltage “Vthp(cur)” of the TFT.Therefore, in the pixel 7 at the “off” state, the gate voltage appliedacross the second TFT 30 is the same in polarity as the second TFT 30 atthe “on” state and is higher than the threshold voltage “Vthp(cur)” ofthe second TFT 30. For example, when the threshold voltage “Vthp(cur)”of the second TFT 30 is −4 V, the gate voltage applied across the secondTFT 30 at the “off” state is set at −3 V.

In the case that the first TFT is of an N type and the second TFT is ofa P type, if the potential of the image signal “data” at the “off” stateis set rather lower than the conventional value, it is possible toreduce the voltage of the image signal “data” and increase the frequencybecause the amplitude of the image signal “data” can be reduced. Evenwhen the potential of the image signal “data” at the “off” state is setrather low, the current at the “put-off” state is very small because inthe P channel type second TFT 30, the potential is rather higher thanthe threshold voltage “Vthp(cur)”. If the voltage applied across theluminescent element 40 is low enough, the driving current flowing intothe element is very small. Accordingly, there are substantially noproblems in turning “off” the luminescent element 40.

In this embodiment, if the potential of the image signal “data” at the“off” state is not required to be higher than the potential of thecommon power supply line “com”, the potential of the common power supplyline “com” can be set rather high. Thus, in this embodiment, thepotential of the common power supply line “com” is set the same as thescanning signal “Sgate” to turn the first TFT “on”. Therefore, thesignal level, which is used as the higher potential of the scanningsignal “Sgate” in the driving circuit at the scanning side, can besupplied, as it is, to the common power supply line “com”. Thus, thenumber of the driving signal levels in use in the display apparatus 1 ofthis embodiment can be reduced to decrease the number of terminals to beinput the driving signal in the display apparatus 1. Further, the numberof power supplies can also be reduced to decrease the power consumptionand reduce the space required.

In this case, since the first TFT 20 is of an N channel type and thesecond TFT 30 is of a P channel type, the potential of the gateelectrode applied across the second TFT 30 of the pixel 7 at “off” stateis set lower than the potential obtained by subtracting the thresholdvoltage “Vthn(sw)” at the first TFT 20 from the potential of thescanning signal “gate” to turn the first TFT 20 “on” state. That is, itis preferable that an absolute value of the potential difference “Voff”between the image signal “data” at turning the pixel 7 “off” state (thepotential of the potential-holding electrode “st”) and the common powersupply line “com” is to be set higher than the threshold voltage“Vthn(sw)” of the first TFT 20, as shown in a formula below, to preventtroubles in the writing operation for the first TFT 20 when selectingthe pixel 7.

Vthn(sw)<|Voff|

In the case of the modified Embodiment 2 where the first TFT 20 is of aP channel type and the second TFT 30 is of an N channel type, as will bedescribed later with reference to FIG. 26 and FIGS. 27 (A) and (B), thepolarities of voltages applied across the first TFT 20 and the secondTFT 30 are inverted by interchanging the relative values of each signaldescribed in Embodiment 2. In this case, if the second TFT 30 is notcompletely turned “off” to turn “off” the luminescent element 40, thereduction of voltage and the increase of frequency of the image signal“data” can be achieved the same as Embodiment 2. If the potential of thecommon power supply line “com” is set equal to the scanning signal“Sgate” to turn the first TFT 20 “on” state, the number of powersupplies can be reduced. In this case, in order to avoid troubles in thewriting operation of the first TFT 20 when selecting the pixel 7, thepotential of the gate electrode applied across the second TFT 30 of thepixel 7 at “off” state is set higher than the potential obtained byaddition the potential of the scanning signal “Sgate” to turn the firstTFT 20 “on” state to the threshold voltage “Vthn(sw)” of the first TFT20.

Embodiment 3 for Carrying Out the Invention

As shown in the FIG. 23 illustrating an equivalent circuit diagram, thisembodiment is an example of structures where the first TFT 20 is of an Nchannel type and the second TFT 30 is of a P channel type in any pixel 7the same as Embodiment 2. In the display apparatus 1 according to thisembodiment, the potential of the common power supply line “com” is sethigher than the potential of the opposite electrode “op” of theluminescent element 40 because the second TFT 30 is of a P channel type.When the second TFT 30 is turned “on”, the current flows from the commonpower supply line “com” to the luminescent element 40, as illustrated bythe arrow “E”. Since this embodiment is similar to Embodiment 2, onlythe differences will be described, and the things in common will beomitted. This embodiment is different from Embodiment 2 in that theholding capacitor “cap” is omitted. By this structure, the potentialchanges of the potential-holding electrode “st” can become larger.

In the case where the first TFT 20 is of a P channel type and the secondTFT 30 is of an N channel type, as will be described later withreference to FIG. 26 and FIGS. 27 (A) and (B), the polarities ofvoltages applied to the first TFT 20 and the second TFT 30 are invertedby interchanging the relative values of each signal described in thatembodiment. Also, in this case, the potential of the selecting pulse ofthe scanning signal is set lower to increase a writing capacity of thefirst TFT 20. The potential of the image signal is raised to raise anemitting luminance by reducing the “on” resistance of the second TFT 30.

A Modified Embodiment 3

In Embodiment 3 described above, the first TFT 20 is of an N channeltype and the second TFT 30 is of a P channel type in any pixel 7,however a structure where the first TFT 20 is of a P channel type andthe second TFT 30 is of an N channel type may also be possible, as shownin the FIG. 26 illustrating an equivalent circuit diagram. In theexemplary structure shown in the figure, the potential of the commonpower supply line “com” is set lower than the potential of the oppositeelectrode “op” of the luminescent element 40, such that when the secondTFT 30 is turned “on”, the current flows from the opposite electrode“op” of the luminescent element 40 to the common power supply line “com”as illustrated by the arrow “F”.

In this structure of the pixel 7, as shown in FIGS. 27 (A) and (B), thepolarity of each driving signal, which has the wave-shapes shown in FIG.24 (A), is inverted.

In Embodiment 3, where the first TFT 20 is of an N channel type and thesecond TFT 30 is of a P channel type, the potential of the common powersupply line “com” may be set lower than the potential of the oppositeelectrode “op” of the luminescent element 40, such that the currentflows from the opposite electrode “op” of the luminescent element 40 tothe common power supply line “com” when the second TFT 30 is turned“on”. Even in this structure, the advantages, which are obtained byforming the first TFT 20 and the second TFT 30 to have an oppositeconductivity, are also obtained. In contrast to this situation, namely,when the first TFT 20 is of a P channel type and the second TFT 30 is ofan N channel type, the potential of the common power supply line “com”may be set higher than the opposite electrode “op” of the luminescentelement 40, such that the current flows from the common power supplyline “com” to the luminescent element 40 when the second TFT 30 isturned “on”, and the advantages obtained by the opposite conductivity ofthe first TFT 20 and second TFT 30 are also obtained.

Embodiment 4 for Carrying Out the Invention

In any of Embodiments 1, 2 and 3, as will be described with reference toFIGS. 28 (A) and (B), it may be formed such that a pulse is suppliedwith one of electrodes of the holding capacitor “cap”. The electrodereceiving the pulse is opposite to the other, which is electricallyconnected to the gate electrode of the second TFT 30. The potential ofthis pulse is opposite to the selecting pulse of the scanning signal“gate”, and the pulse is supplied the electrode with a delay behind theselecting pulse.

this example, as shown in FIG. 28 (A), one of both of the electrodes ofthe holding capacitor “cap”, which is opposite to the one, which iselectrically connected to the gate electrode of the second TFT 30through the potential-holding electrode “st”, is formed by the capacitorline “cline”, which is extended in parallel with the scanning line“gate”.

As shown in FIG. 28 (B), this capacitor line “cline” is formed such thatthe potential “stg” is supplied to the capacitor line “cline” with adelay behind the selecting pulse “Pgate” of the scanning signal “Sgate”.The potential “stg” supplied to the capacitor line “cline” includes thepulse signal “Pstg”, and the polarity of which is opposite to thepolarity of the selecting pulse.

After the selecting pulse becomes non-selective, the pulse signal “Pstg”shifts the potential of the image signal “data” utilizing a capacitivecoupling of the holding capacitor “cap”. Therefore, signals are held inthe holding capacitor “cap” at the “off-state” pixel 7, corresponding tothe potential obtained by adding the potential of the pulse signal“Pstg” to the potential of the image signal “data”. Due to the high “on”resistance of the first TFT 20, it is difficult to completely write thesignals in the higher potential side of the image signals “data” withina limited time. In the case of the example, a shortage of the writingcapacity results in no emitting of the pixel. However, in accordancewith Embodiment 4, it is possible to supplement the writing of the imagesignal “data” to the holding capacitor “cap”, even though the maximumrange of the potential in the driving signal is not expanded.

When the pulse signal “Pstg” is stored in the capacitor line “cline”, asshown in FIG. 29, the “cline” is extracted from the driving circuit 4 inthe scanning side. At the same time, in the driving circuit 4 of thescanning side, the output signal from a shift resistor 401 is outputtedto any of gate stages, as the scanning signal “Sgate”, through NAND gatecircuit and an inverter. On the other hand, the output signal from theshift resistor 401 is outputted to the capacitor line “cline”, throughthe NAND gate circuit and the two staged inverter, with a delay behindthe scanning signal, shifting the power level in the higher potentialside from “Vddy” to “Vccy”, as shown in FIG. 30.

In the above mentioned embodiments and their modified embodiments,concerning the case that the holding capacity is added, the type ofluminescent element having the capacitor line “cline” was described.However, since this embodiment is not limited to this structure havingthe capacitor line “cline”, it is also possible to form one of theelectrodes of the holding capacitor by the adjacent gate line. As anexample of these structures, FIGS. 34 (A) and (B) illustrate a circuitblock diagram and a voltage waveform chart of the gate electrode in thedirection of the scanning of the gate line respectively. There is anadvantage that it is possible to avoid taking the trouble to form thecapacitor line “cline”, by forming the gate line, which is adjacent tothe pixels, as the one of the electrodes of the holding capacitor.

Other Embodiments for Carrying Out the Invention

In any of the aforementioned embodiments, a region in the ampere-voltcharacteristic where the second TFT 30 is operated has not beendescribed. If the second TFT 30 is operated at the saturated region, itis possible to prevent an abnormal current flow in the luminescentelement 40, utilizing a weak constant-current characteristic. Forexample, the organic semiconductor film, etc., forming the luminescentelement 40 can possibly have pinhole defects, even though this does notcause a complete short circuit across the electrodes of the luminescentelement 40, due to the restricted current in the luminescent elementwith the defect.

If the second TFT 30 is operated at the linear region, it is possible toprevent the display operation from affecting by unevenness of thethreshold voltage.

In addition, the TFT may be formed in a bottom gate type as well as in atop gate type, and the production method is not limited to a lowtemperature process in producing the TFT.

INDUSTRIAL APPLICABILITY

In the display apparatus in accordance with the claims 1 to 7 of thepresent invention, as described above, the gate voltage of the secondTFT at the time of the “on” corresponds to the difference between thepotential of the gate electrode (the potential of the image signal), andone of the potential of the common power supply line and the potentialof the pixel electrode. Therefore the display apparatus is formed suchthat the relative potential values of the common power supply line andthe potential-holding electrode are set depending on the conduction typeof the second TFT, and such that the gate voltage of the second TFTcorresponds to the difference between the potential of the common powersupply line and the potential of the potential-holding electrode.

or example, if the second TFT is of an N channel type, the potential ofthe common power supply line is set lower than that of the oppositeelectrode of the luminescent element. Since this potential of the commonpower supply line can be set low enough, different from the potential ofthe pixel electrode, the large “on” current in the second TFT and ahigh-luminance display can be obtained. When the pixel is turned “on”,if the high gate voltage of the second TFT can be obtained, thepotential of the image signal at the time can be reducedcorrespondingly. Thus, the amplitude of the image signal can be reducedto lower the driving voltage in the display apparatus. Therefore, thereis an advantage that the problem of withstanding voltage, which concernseach element formed by a film, is not encountered in conjunction withthe power consumption being reduced.

In the display apparatus in accordance with the Claims 7 to 11 of thepresent invention, since the first TFT and the second TFT are formed ofthe opposite conduction types, the pulse of the scanning signal toselect the pixel is of opposite polarity to the potential of the imagesignal to turn “on” the luminescent element. When using the potential ofthe potential-holding electrode at the time of the “on” (the potentialof the image signal to turn “on”) as a reference, the potentialcorresponding to the higher potential of the scanning signal and thepotential of the common power supply line are the same in theirpolarity. Therefore, if the potential of the potential-holding electrodeat the time of “on” (the potential of the image signal to turn “on”) ischanged, both of the gate voltage of the first TFT and that of thesecond TFT change correspondingly, in the same direction and by the sameamount. Accordingly, if the potential of the image signal to turn “on”is shifted to the direction for reducing the resistance of the first TFTat the time of the “on”, within the driving voltage range of the displayapparatus, higher speed operation of the display can be offered. Since,at the same time, it means that the potential of the image signal toturn “on” is shifted to the direction for reducing the resistance of thefirst TFT at the “on”, as a result, a luminance in the display can beimproved in conjunction with the above described advantages. Thus, thereduced driving voltage and the improved quality of the display can beaccomplished, simultaneously.

Further, in the display apparatus in accordance with the Claim 11 or 12of the present invention, one of the holding capacitor electrodes, whichis opposite to the one that is electrically connected to the second gateelectrode of the second TFT, is supplied with a pulse having a potentialpolarity opposite to the selecting pulse of the scanning signal, with adelay behind the selecting pulse. Therefore, the writing of the imagesignal into the holding capacitor can be supplemented. Thus, thepotential of the image signal applied to the gate electrode of thesecond TFT can be shifted to the direction of a higher luminance withoutincreasing the amplitude of the image signal.

1. A display device comprising: a scanning line; a data line; a powersupply line; and a pixel, the pixel comprising: a first transistor towhich a scanning signal is supplied through the scanning line; a secondtransistor that is drive-controlled according to an image signalsupplied through the first transistor from the data line; and aluminescent element that emits due to a driving current that flowsbetween a pixel electrode and an opposite electrode when the pixelelectrode is electrically connected to the power supply line through thesecond transistor, the second transistor being an N-channel type, apotential of the power supply line being set at a lower level than apotential of the opposite electrode, and a potential of the image signalthat is supplied from the data line to a pixel to be turned on is lowerthan or equal to the potential of the opposite electrode.
 2. A displaydevice comprising: a scanning line; a data line; a power supply line;and a pixel, the pixel comprising: a first transistor to which ascanning signal is supplied through the scanning line; a secondtransistor that is drive-controlled according to an image signalsupplied through the first transistor from the data line; and aluminescent element that emits due to a driving current that flowsbetween a pixel electrode and an opposite electrode when the pixelelectrode is electrically connected to the power supply line through thesecond transistor, the second transistor being an N-channel type, apotential of the power supply line being set at a lower level than apotential of the opposite electrode, and a potential that is supplied toa gate electrode of the second transistor from the image signal that issupplied from the data line to a pixel to be turned on is lower than orequal to a potential of the opposite electrode.
 3. The display device asset forth in claim 1, the potential of the image signal that is suppliedfrom the data line to a pixel to be turned off being higher than orequal to the potential of the power supply line.
 4. The display deviceas set forth in claim 1, a potential that is supplied to a gateelectrode of the second transistor from the image signal that issupplied from the data line to a pixel to be turned off is higher thanor equal to a potential of the power supply line.
 5. The display deviceas set forth in claim 1, the first transistor being a P-channel type. 6.The display device as set forth in claim 1, the first transistor beingan N-channel type.
 7. The display device as set forth in claim 1, thepixel including a holding capacitor that holds the image signal that issupplied through the first transistor from the data line, and apotential held by the holding capacitor being supplied to the gateelectrode of the second transistor.
 8. A display device comprising: ascanning line; a data line; a power supply line; a pixel, the pixelhaving: a first transistor that is supplied with a selecting pulse of ascanning signal through the scanning line; a holding capacitor thatholds an image signal supplied through the data line and the firsttransistor, the holding capacitor having a first electrode and a secondelectrode; a second transistor of which conduction state is controlledby the image signal, a gate electrode of the second transistor beingelectrically connected to the second electrode; and a luminescentelement provided between a pixel electrode and an opposite electrodeopposed to the pixel electrode, the luminescent element being able toemit light due to driving current that flows between the pixel electrodeand the opposite electrode when the pixel electrode is electricallyconnected to the power supply line through the second transistor, apotential of the gate electrode of the second transistor being able tobe shifted by supplying to the first electrode of the holding capacitora predetermined signal after the selecting pulse becomes non-selective.9. A display device comprising: a scanning line; a data line; a powersupply line; and a pixel, the pixel comprising: a first transistor towhich a scanning signal is supplied through the scanning line; a holdingcapacitor that holds an image signal which is supplied from the dataline through the first transistor and is provided with first and secondelectrodes; a second transistor that is drive-controlled according tothe image signal supplied through the first transistor from the dataline, and of which a gate electrode is connected to the secondelectrode; and a luminescent element that emits light due to a drivingcurrent that flows between a pixel electrode and an opposite electrodewhen the pixel electrode is electrically connected to the power supplyline through the second transistor, a pulse that is delayed compared toa selection pulse of the scanning signal, and of which a potential isoscillated in a reverse direction, being supplied to the firstelectrode.
 10. The display device as set forth in claim 1, theluminescent element including an organic semiconductor film.
 11. Thedisplay device as set forth in claim 1, the second transistor beingconstituted so as to be operated in a saturation area.
 12. The displaydevice as set forth in claim 1, the second transistor being constitutedso as to be operated in a linear area.